1. Field of Invention
The present invention, in various embodiments, relates to an in situ relaxation modulus sensor for viscoelastic materials, structures incorporating such sensors and a method for use of such a sensor.
2. Discussion of Related Art
The relaxation modulus of a viscoelastic material is a coefficient describing the material's property of releasing or “relaxing” over time when under constant deformation at a constant temperature. Relaxation modulus is a time-dependent material property. The force required to hold a viscoelastic material in constant deformation, while at a constant temperature, diminishes over time. The relaxation modulus of a material is indicative of the mechanical stiffness of the material.
The stress relaxation of an elastomeric material may be measured using conventional test equipment such as the testing apparatus disclosed in U.S. Pat. No. 3,693,421 to Karper et al. A sample of the material is located within a recess of a stator die member, and a piston is used to maintain a predetermined regulated pressure of a conical die member on the sample. The conical die member is rotated a small preselected degree, which in turn exerts a predetermined torque on the sample. A load cell on the mechanism driving the conical die member measures the relaxation which occurs within the specimen.
Another conventional method of measuring the relaxation modulus of a test piece is using the testing device of U.S. Pat. No. 5,394,753 to Moriyoshi. A test piece is attached between two relatively displaceable members and is compressed and curved when a drive means moves the members relative to one another. A load cell is positioned between the drive means and the displaceable members. The load cell may be used to determine the load applied to the test piece.
These conventional methods of measuring stress relaxation and relaxation modulus require a sample of the desired material to be taken from the application (e.g., the structure incorporating the material) or from stock material. The sample is then deformed for testing. Removing the sample of the material from the application requires either disassembly and repair of the application, or the application must be sacrificed in order to conduct the testing. For example, in order to conventionally test rocket motors for the effects of chemical aging on both the propellant and the rocket motor liner, it is necessary to dissect motors, cut samples of the propellant and liner, and perform destructive laboratory tests to monitor the changes. The measured motors are destroyed and it is necessary to assume that they are representative of the remaining motors in the fleet. However, chemical aging trends may be masked by differences between motors, such as the chemical composition of the propellant, variations in the liner composition, or by environmental factors such as storage location, storage temperature, temperature change, humidity or exposure to contaminants. A faulty assumption regarding the condition of an untested rocket motor could potentially lead to catastrophic failure and possible loss of life. It would be advantageous to monitor and test the mechanical properties of the propellant and liner non-destructively in the individual rocket motors. It would be desirable to determine the aging trends for each individual motor, rather than extrapolating trends from a single test motor.
Therefore it would be advantageous to provide a device and method for non-destructive testing of viscoelastic materials. Non-destructive testing of the material properties of an item, for example the relaxation modulus, may ameliorate one cost of the testing (the destruction of the item) and provide accurate test results.